When Did The Universe Become Transparent To Light?

A young, star-forming region found within our own Milky Way. Note how the material around the stars gets ionized, and over time becomes transparent to all forms of light. Until that happens, however, the surrounding gas absorbs the radiation, emitting light of its own of a variety of wavelengths. In the early Universe, it takes hundreds of millions of years for the Universe to fully become transparent to light.

If you want to see what's out there in the Universe, you first have to be able to see. We take for granted, today, that the Universe is transparent to light, and that the light from distant objects can travel unimpeded through space before reaching our eyes. But it wasn't always this way.

In fact, there are two ways that the Universe can stop light from propagating in a straight line. One is to fill the Universe with free, unbound electrons. The light will then scatter with the electrons, bouncing off in a randomly-determined direction. The other is to fill the Universe with neutral atoms that can clump and cluster together. The light will then be blocked by this matter, the same way that most solid objects are opaque to light. Our actual Universe does both of these, and won't become transparent until both obstacles are overcome.

Neutral atoms were formed just a few hundred thousand years after the Big Bang. The very first stars began ionizing those atoms once again, but it took hundreds of millions of years of forming stars and galaxies until this process, known as reionization, was completed.

THE HYDROGEN EPOCH OF REIONIZATION ARRAY (HERA)

In the earliest stages of the Universe, the atoms that make up everything we know of weren't bound together in neutral configurations, but rather were ionized: in the state of a plasma. When light travels through a dense-enough plasma, it will scatter off of the electrons, being absorbed and re-emitted in a variety of unpredictable directions. So long as there are enough free electrons, the photons streaming through the Universe will continue to be kicked around at random.

There's a competing process occurring, however, even during these early stages. This plasma is made of electrons and atomic nuclei, and it's energetically favorable for them to bind together. Occasionally, even at these early times, they do exactly that, with only the input from a sufficiently energetic photon capable of splitting them apart once again.

As the fabric of the Universe expands, the wavelengths of any radiation present get stretched as well. This causes the Universe to become less energetic, and makes many high-energy processes that occur spontaneously at early times impossible at later, cooler epochs. It requires hundreds of thousands of years for the Universe to cool enough so that neutral atoms can form.

E. Siegel / Beyond The Galaxy

As the Universe expands, however, it not only gets less dense, but the particles within it get less energetic. Because the fabric of space itself is what's expanding, it affects every photon traveling through that space. Because a photon's energy is determined by its wavelength, then as that wavelength gets stretched, the photon gets shifted — redshifted — to lower energies.

It's only a matter of time, then, until all the photons in the Universe drop below a critical energy threshold: the energy required to knock an electron off of the individual atoms that exist in the early Universe. It takes hundreds of thousands of years after the Big Bang for photons to lose enough energy to make the formation of neutral atoms even possible.

At early times (left), photons scatter off of electrons and are high-enough in energy to knock any atoms back into an ionized state. Once the Universe cools enough, and is devoid of such high-energy photons (right), they cannot interact with the neutral atoms. Instead, they simply free-stream through space indefinitely, since they have the wrong wavelength to excite these atoms to a higher energy level.

E. Siegel / Beyond the Galaxy

Many cosmic events happen during this time: the earliest unstable isotopes radioactively decay; matter becomes more energetically important than radiation; gravitation begins pulling matter into clumps as the seeds of structure start growing. As the photons become more and more redshifted, another barrier to neutral atoms appears: the photons emitted when electrons bind to protons for the first time. Every time an electron successfully binds with an atomic nucleus, it does two things:

It emits an ultraviolet photon, because atomic transitions always cascade down in energy levels in a predictable fashion.

It gets bombarded by other particles, including the billion-or-so photons that exist for every electron in the Universe.

Every time you form a stable, neutral atom, it emits an ultraviolet photon. Those photons then continue on, in a straight line, until they encounter another neutral atom, which they then ionize.

When free electrons recombine with hydrogen nuclei, the electrons cascade down the energy levels, emitting photons as they go. In order for stable, neutral atoms to form in the early Universe, they have to reach the ground state without producing an ultraviolet photon that could potentially ionize another identical atom.

Brighterorange & Enoch Lau/Wikimdia Commons

There's no net addition of neutral atoms through this mechanism, and hence the Universe cannot become transparent to light through this pathway alone. There's another effect that comes in, instead, that dominates. It's extremely rare, but given all the atoms in the Universe and the more-than-100,000 years it takes for atoms to finally and stably become neutral, it's an incredible and intricate part of the story.

Most times, in a hydrogen atom, when you have an electron occupying the first excited state, it simply drops down to the lowest-energy state, emitting an ultraviolet photon of a specific energy: a Lyman alpha photon. But about 1 time in 100 million transitions, the drop-down will occur through a different path, instead emitting two lower-energy photons. This is known as atwo-photon decay or transition, and is what is primarily responsible for the Universe becoming neutral.

When you transition from an "s" orbital to a lower-energy "s" orbital, you can on rare occasion do it through the emission of two photons of equal energy. This two-photon transition occurs even between the 2s (first excited) state and the 1s (ground) state, about one time out of every 100 million transitions.

R. Roy et al., Optics Express 25(7):7960 · April 2017

When you emit a single photon, it almost always collides with another hydrogen atom, exciting it and eventually leading to its reionization. But when you emit two photons, it's extraordinarily unlikely that both will hit an atom at the same time, meaning that you net one additional neutral atom.

This two-photon transition, rare though it is, is the process by which neutral atoms first form. It takes us from a hot, plasma-filled Universe to an almost-equally-hot Universe filled with 100% neutral atoms. Although we say that the Universe formed these atoms 380,000 years after the Big Bang, this was actually a slow, gradual process that took about 100,000 years on either side of that figure to complete. Once the atoms are neutral, there is nothing left for the Big Bang's light to scatter off of. This is the origin of the CMB: the Cosmic Microwave Background.

A Universe where electrons and protons are free and collide with photons transitions to a neutral one that's transparent to photons as the Universe expands and cools. Shown here is the ionized plasma (L) before the CMB is emitted, followed by the transition to a neutral Universe (R) that’s transparent to photons. The scattering between electrons and electrons, as well as electrons and photons, can be well-described by the Dirac equation, but photon-photon interactions, which occur in reality, are not.

Amanda Yoho

This marks the first time that the Universe becomes transparent to light. The leftover photons from the Big Bang, now long in wavelength and low in energy, can finally travel freely through the Universe. With the free electrons gone — bound up into stable, neutral atoms — the photons have nothing to stop them or slow them down.

But the neutral atoms are now everywhere, and they serve an insidious purpose. While they may make the Universe transparent to these low-energy photons, these atoms will clump together into molecular clouds, dust, and collections of gas. Neutral atoms in these configurations might be transparent to low-energy light, but the higher-energy light, like that emitted by stars, gets absorbed by them.

An illustration of the first stars turning on in the Universe. Without metals to cool down the stars, only the largest clumps within a large-mass cloud can become stars. Until enough time has passed for gravity to affect larger scales, only the small-scales can form structure early on, and the stars themselves will see their light unable to penetrate very far through the opaque Universe.

NASA

When all of the atoms in the Universe are now neutral, they do an amazingly good job of blocking starlight. The same long-awaited configuration that we required to make the Universe transparent now makes it opaque again to photons of a different wavelength: the ultraviolet, optical, and near-infrared light produced by stars.

In order to make the Universe transparent to this other type of light, we'll need to ionize them all again. This means that we need enough high-energy light to kick the electrons off of the atoms they're bound to, which requires an intense source of ultraviolet emission.

In other words, the Universe needs to form enough stars to successfully reionize the atoms within it, rendering the tenuous, low-density intergalactic medium transparent to starlight.

This four-panel view shows the Milky Way's central region in four different wavelengths of light, with the longer (submillimeter) wavelengths at top, going through the far-and-near infrared (2nd and 3rd) and ending in a visible-light view of the Milky Way. Note that the dust lanes and foreground stars obscure the center in visible light, but not so much in the infrared.

We see this even in our own galaxy: the galactic center cannot be seen in visible light. The galactic plane is rich in neutral dust and gas, which is extremely successful at blocking the higher-energy ultraviolet and visible light, but infrared light goes clear through. This explains why the cosmic microwave background won't get absorbed by neutral atoms, but starlight will.

Thankfully, the stars that we form can be massive and hot, where the most massive ones are much more luminous and hotter than even our Sun. Early stars can be tens, hundreds, or even a thousand times as massive as our own Sun, meaning they can reach surface temperatures of tens of thousands of degrees and brightnesses that are millions of times as luminous as our Sun. These behemoths are the biggest threat to the neutral atoms spread throughout the Universe.

The first stars in the Universe will be surrounded by neutral atoms of (mostly) hydrogen gas, which absorbs the starlight. The hydrogen makes the Universe opaque to visible, ultraviolet, and a large fraction of infrared light, but long wavelength light, such as radio-light, can transmit unimpeded.

Nicole Rager Fuller / National Science Foundation

What we need to happen is for enough stars to form that they can flood the Universe with a sufficient number of ultraviolet photons. If they can ionize enough of this neutral matter filling the intergalactic medium, they can clear a path in all directions for starlight to travel unimpeded. Moreover, it has to occur in sufficient amounts that the ionized protons and electrons can't get back together again. There is no room for Ross-and-Rachel style shenanigans in the effort to reionize the Universe.

The first stars make a small dent in this, but the earliest star clusters are small and short-lived. For the first few hundred million years of our Universe, all the stars that form can barely make a dent in how much of the matter in the Universe remains neutral. But that begins to change when star clusters merge together, forming the first galaxies.

An illustration of CR7, the first galaxy detected that was thought to house Population III stars: the first stars ever formed in the Universe. JWST will reveal actual images of this galaxy and others like it, and will be able to make measurements of these objects even where reionization has not yet completed.

ESO/M. Kornmesser

As large clumps of gas, stars, and other matter merge together, they trigger a tremendous burst of star formation, lighting up the Universe as never before. As time goes on, a slew of phenomena take place all at once:

the regions with the largest collections of matter attract even more early stars and star clusters towards them,

the regions that haven't yet formed stars can begin to,

and the regions where the first galaxies are made attract other young galaxies,

all of which serves to increase the overall star formation rate.

If we were to map out the Universe at this time, what we'd see is that the star formation rate increases at a relatively constant rate for the first few billion years of the Universe's existence. In some favorable regions, enough of the matter gets ionized early enough that we can see through the Universe before most regions are reionized; in others, it may take as long as two or three billion years for the last neutral matter to be blown away.

If you were to map out the Universe's neutral matter from the start of the Big Bang, you would find that it starts to transition to ionized matter in clumps, but you'd also find that it took hundreds of millions of years to mostly disappear. It does so unevenly, and preferentially along the locations of the densest parts of the cosmic web.

Schematic diagram of the Universe's history, highlighting reionization. Before stars or galaxies formed, the Universe was full of light-blocking, neutral atoms. While most of the Universe doesn't become reionized until 550 million years afterwards, some regions will achieve full reionization earlier and others won't achieve it until later. The first major waves of reionization begin happening at around 250 million years of age, while a few fortunate stars may form just 50-to-100 million years after the Big Bang. With the right tools, like the James Webb Space Telescope, we may begin to reveal the earliest galaxies.

S. G. Djorgovski et al., Caltech Digital Media Center

On average, it takes 550 million years from the inception of the Big Bang for the Universe to become reionized and transparent to starlight. We see this from observing ultra-distant quasars, which continue to display the absorption features that only neutral, intervening matter causes. But reionization doesn't happen everywhere at once; it reaches completion at different times in different directions and at different locations. The Universe is uneven, and so are the stars and galaxies and clumps of matter that form within it.

The Universe became transparent to the light left over from the Big Bang when it was roughly 380,000 years old, and remained transparent to long-wavelength light thereafter. But it was only when the Universe reached about half a billion years of age that it became fully transparent to starlight, with some locations experiencing transparency earlier and others experiencing it later.

To probe beyond these limits requires a telescope that goes to longer and longer wavelengths. With any luck, the James Webb Space Telescope will finally open our eyes to the Universe as it was during this in-between era, where it's transparent to the Big Bang's glow but not to starlight. When it opens its eyes on the Universe, we may finally learn just how the Universe grew up during these poorly-understood dark ages.